subsurface geology and coal resources of the pennsylvanian system ...

10 ILLIN018 BASIN COAL RESOURCES
difficult the stratigraphic correlation and
classification in such areas. The No. 6 coal
bed is very nearly continuous in the Illinois
basin, particularly in the southern half of
the basin which includes the counties represented
in the accompanying series of reports.
This coal bed is used in all the
accompanying reports as the predominant
key bed.
The Herrin limestone, the caprock of the
No. 6 coal bed, is commonly so closely associated
with the coal bed that together they
constitute what might be regarded as dual
key beds. This limestone is known as the
Brereton limestone in western Illinois5nd
as the Providence limestone in western
Ment~cky,~ from which area the use has
spread more or less into southwestern Indiana
and southeastern Illinois.
In some parts of southern Illinois, particularly
in western Franklin and Williamson
counties, the No. 6 coal bed and the caprock
are separated by as much as 60 feet oi:
shale or siltstone, and this separation of the
beds may make the identification of both
somewhat uncertain.
Other stratigraphic units that are used
as key beds in the Illinois basin, or in large
portions of the basin, lack the widespread
distribution of the dual key beds, Herrin
(No. 6) coal bed and limestone. Four
such units are the Millersville limestone,
the Shoal Creek limestone, the West Franklin
limestone, and Harrisburg (No. 5) coal
bed. Another unit of somewhat less certain
position and identity is the "No. 7"
coal bed.
The Millersville limestone5 is one of the
thickest (30 to 50 feet) Pennsylvanian
limestones in the northern part of the Illinois
basin. South of Efingham County it
becomes thinner or disappears entirely, thus
losing its value as a key bed. Its position
is approximately 600 feet above No. 6 coal
bed.
The limestone designated as Shoal Creek
is believed to be the same as the Shoal Creek
Savage. T. E., Significant breaks and overlaps in the
Pennsylvanian I ocks of Illinois : Amer. Jour. Science, 5th
XI.. vol. 14. pp. 307-316, 1927.
Glenn, L. C.. Geology and coals of Webster County:
Kentucky Geol. Survey, ser. 6, vol. 5, p. 98, 1922.
Taylor, Earle F., and Cady, Gilbert H., Structure of
the Millersville limestone in the north part of the Illinois
basin: Illinois Geol. Survey Rept. Inv. 93, p. 22, 1944.
limestone that crops out along the stream
of that name in Clinton County.Vt ranges
from 450 feet above No. 6 coal bed in the
southern part of the basin to about 280 feet
in Clay County, and lies 150 to 200 feet
below the Millersville limestone. There
are fairly wide variations in these intervals,
probably in the order of 50 feet. The Shoal
Creek limestone underlies the west side of
the Illinois basin fairly continuously, but
toward the east side of Clay, Wayne, and
Hamilton counties it becomes thin or disappears.
In the areas where the Shoal
Creek limestone is only locally present it
does not provide a satisfactory key bed.
Wh'ere characteristically developed in the
Illinois basin, it is usually 5 to 10 feet
thick. It generally overlies a black shale,
with a thin coal bed commonly present 20
to 40 feet below.
The West Franklin limestone lies about
250 feet above the No. 6 coal bed and about
200 feet below the position of the Shoal
Creek limestone. However, the two limestones
do not appear to be coextensive, the
western margin of the recognizable West
Franklin limestone being not far from the
eastern margin of the recognizable Shoal
Creek limestone. Consequently both limestones
are not usually available as key beds
in the same counties, or at least not in the
same parts of an individual county. The
West Franklin limestone thickens to the
east, and in southern Indiana it commonly
consists of three beds with intervening shale
of variegated ~oloring.~
In drill holes in the Illinois basin one,
two, or three benches may be penetrated
or reported in the logs, and when only one
or two benches are reported it is impossible
to know with certainty which ones are represented.
The middle bench is usually the
thickest where three benches are present,
so a single bench or the thickest of two
benches is generally regarded as representing
the middle bench. Red shale is thought
to be more characteristic of the interval
between the two lower benches than of the
interval between the two upper benches or
Udden, Jon A., Notes on the Shoal Creek limestone:
Tllinois Geol. Survey Bull. 8, pp. 117-126, 1907.
'See "Subsurface geology of Gallatin County," for de-
5cription at two localities in southern Indiana, this report,
p. 69.